In pumps of this type pneumatically conveyable bulk material is fed from an infeed container into a conveyor line in which a higher gas pressure prevails than at the feed area. In order that the gas cannot pass from the conveyor line through the pump to the infeed area and escape at that point, a nonreturn valve is provided. The nonreturn valve is not opened until an appropriate pressure of material conveyed by the pump has developed during the operation of the pump. Since the chamber in front of the nonreturn valve (viewed in the direction of conveying) is then filled with material, the gas cannot escape to the rear.
The nonreturn valve can be a simple flap with an appropriate fastening with a pivotable lever if the screw conveyor is mounted only at its front end, i.e., in the vicinity of the infeed area (German Federal Republic application DE-AS No. 25 07 678). However, such a pump has the disadvantage that, particularly at high rpm's, the free end of the screw conveyor can vibrate, which vibrations can only be overcome with difficulty and great technical expense. In any case, it is simpler and more reliable to mount the screw conveyor at each end.
Then, however, there is naturally the problem that a simple nonreturn valve can no longer be arranged at the outlet end of the screw conveyor. This outlet valve was therefore positioned at the side of the screw conveyor (German Federal Republic application DE-AS No. 27 51 773). However, this results in the following disadvantage. The material to be conveyed is compressed in the vicinity of the nonreturn valve. This compression might be desirable, in order to achieve said seal against backflowing gas. If this compressed material must be deflected laterally too, possible by 90.degree., additional energy is required for this. There is also a greater load on the bearings and very great wear. These disadvantages can be reduced by an enlarged shaft diameter with ribs added in the discharge area. Then, however, there is no longer the possibility of making the end areas of the screw conveyor spiral, which wear down especially rapidly, exchangeable.
There is also the possibility of positioning the rear mounting of the screw conveyor in the current of material (European application EP - OS No. 00 41 279). For this, the screw conveyor housing is lengthened somewhat, the rear bearing is positioned on supports between which the material flows and the nonreturn valve is provided behind this mounting. Then, however, there are problems in as far as it cannot be prevented that very fine granular material can penetrate into the rear bearing and gradually destroy it.
Finally, the attempt was made to manufacture the nonreturn valve from two elements which surround the screw conveyor shaft in the vicinity of the outlet opening (French Pat. No. 2,110,816). In this instance, however, a good seal is not achieved, as several slits remain open between the individual closing elements. On the other hand, if the slit between the individual flap elements is made small, there is the danger that these elements can jam against each other. Also, vortices will necessarily occur at the additional edges of the flaps which result in greater wear, as the slug of bulk material conveyed by the screw conveyor rotates at least partially together with the screw conveyor.